In recent years, digital cameras that capture two-dimensional images by projecting an image of a subject through a lens onto a camera sensor that is a group of a large number of pixels made of a semiconductor material and then measuring the amount of light with which each pixel is exposed have become more common.
Moreover, in cameras, in order to project a focused image of the subject onto the camera sensor or the surface of film, the distance between the lens and the camera sensor is required to be adjusted such that the point light sources that is a group of self-emitting or reflected point light sources included in the image of the subject create blur spots of less than or equal to a prescribed diameter on the camera sensor or the film surface. However, most cameras have an autofocus feature that handles this process automatically according to the distance between the subject and the camera.
Furthermore, “blur spots of less than or equal to a prescribed diameter” refers to blur spots of a diameter less than or equal to the resolution of the human eye when the captured image is viewed from the distance of distinct vision, that is, when viewed from a distance at which the human eye can focus on and see the image clearly without having to focus in a particularly strenuous manner. When viewing most typically photographed images, diameters of approximately 1/1000 to 1/1500 of the diagonal of the image represent the limit of resolution of the human eye.
In order to autofocus, instead of performing a manual operation by the user to bring the desired subject within the screen into focus, the camera first needs to focus on the object positioned in the center of the screen as the user's intended subject within the screen, and the subject must always be brought into the center of the screen in this case. In other words, there are situations in which it is difficult to focus the image if the subject moves away from the center of the screen.
One example of a new autofocus technology for automatically focusing on a user's intended subject is the autofocus device disclosed in Japanese Unexamined Patent Application Publication No. H5-80248 (Patent Literature 1). In this autofocus device, the subject to be captured is identified within an area of focus, and the focal position of the identified subject is automatically detected to bring that subject into focus. The device includes a movement detection unit that calculates, on a per-frame basis, the entropy of images within each small area of a region of interest that is constituted by a plurality of such small areas and then detects changes in the entropy of each small area between frames in order to detect movement of the subject. This technology therefore allows the area of focus to be moved to track the movement of the subject in accordance with the detection results from the movement detection unit.
In Patent Literature 1, “entropy” is a value that quantifies randomness, and in the operation in Patent Literature, the entropy of the subject within the screen targeted in advance by the user is obtained as a value for identifying the subject by recognizing the randomness of the output data from the portion of the camera sensor onto which the subject to be captured is projected at the position on the camera sensor onto which the image of the subject is projected, and when the subject moves within the screen, the entropy of each screen location is calculated, whereby the optical image projected onto the region with the closest entropy to the original entropy of the subject is recognized as the subject, and then the focus is brought onto that subject.
Furthermore, the types of subjects that users typically capture images of are limited to some extent. One specific example of such a common subject is peoples' faces, and for this use case, there are already a variety of camera products on the market that can extract a person's face as the subject and focus on that subject automatically.